Symmetrical dyes with large two-photon absorption cross-sections

ABSTRACT

A two-photon absorbing chromophore of the formula 
     
         E--Ar--E 
    
     wherein the Ar core is selected from the group consisting of ##STR1## wherein R 1  and R 2  are alkyl groups having 8 to 12 carbon atoms, and wherein R 1  and R 2  are the same or different, and wherein E is a moiety selected from the group consisting of ##STR2##

RIGHTS OF THE GOVERNMENT

The invention described herein may be manufactured and used by or for the Government of the United States for all governmental purposes without the payment of any royalty.

BACKGROUND OF THE INVENTION

In ordinary fluorescence microscopy, defocused images outside the depth of focus are superimposed on an image formed on the focal plane. This globally lowers the contrast of microscopic image, which makes determination of fluorescence intensity difficult.

Confocal microscopy offers several advantages over conventional microscopy. The shallow depth of field, generally about 0.5 to 1.5 μm, of confocal microscopes allows information to be collected from a well defined optical section rather than from most of the specimen as in conventional light microscopy. Consequently, out-of-focus fluorescence is virtually eliminated, which results in an increase in contrast, clarity and detection.

In a point scanning confocal system, the microscope lens focus the laser light on one point in the specimen at a time, i.e., the focal point. The laser moves rapidly from point to point to produce a scanned image. Very little of the laser light falls on other points in the focal plane. Both fluorescent and reflected light from the sample pass back through the microscope. The microscope and the optics of the scanner compartment focus the fluorescent light emitted from the focal point to a secont point, called the confocal point. A pinhole aperature, located at the confocal point, allows light from the focal point to pass through to a detector. Light emitted from outside the focal point is rejected by the aperature. Accordingly, only the image near the focal plane inside the sample is obtained as a microscopic image.

In two-photon absorption excitation type laser scanning fluorescence microscopy, a laser beam forms an optical spot having a high energy density and the optical spot three-dimensionally scans the inside of a sample in the same manner as in confocal laser scanning fluorescence microscopy. Because of the arrangement, fluorescence due to excitation based on two-photon absorption appears only from a point where the optical spot is located inside the sample but no fluorescence due to excitation based on two-photon absorption appears from other portions. Therefore, there appears no defocused image other than one on the focal plane, which improves the contrast of the microscopic image.

Two-photon excitation is made possible by the combination of (a) the very high, local, instantaneous intensity provided by the tight focusing available in a laser scanning microscope, wherein the laser can be focused to diffraction-limited waist of less than 1 micron in diameter, and (b) the temporal concentration of a pulsed laser. A high intensity, long wavelength, monochromatic light source which is focusable to the diffraction limit such as a colliding-pulse, mode-locked dye laser, produces a stream of pulses, with each pulse having a duration of about 100 femtoseconds (100×10⁻¹⁵ seconds) at a repetition rate of about 80 MHz. These subpicosecond pulses are supplied to the microscope, for example by way of a dichroic mirror, and are directed through the microscope optics to a specimen, or target material, located at the object plane of the microscope. Because of the high instantaneous power provided by the very short duration intense pulses focused to the diffraction limit, there is an appreciable probability that a fluorophore (a fluorescent dye), contained in the target material, and normally excitable by a single high energy photon having a short wavelength, typically ultraviolet, will absorb two long wavelength photons from the laser source simultaneously. This absorption combines the energy of the two photons in the fluorophore molecule, thereby raising the fluorophore to its excited state. When the fluorophore returns to its normal state, it emits light, and this light then passes back through the microscope optics to a suitable detector.

The probability of absorption of two long wavelength photons from the laser source simultaneously is dependent upon the two-photon cross-section of the dye molecule.

Accordingly, it is an object of the present invention to provide chromophores having large two-photon cross-sections.

Other objects and advantages of the present invention will be apparent to those skilled in the art.

SUMMARY OF THE INVENTION

In accordance with the present invention there are provided symmetrical chromophores of the formula:

    E--Ar--E

wherein the Ar core is selected from the group consisting of ##STR3## wherein R₁ and R₂ are alkyl groups having 8 to 12 carbon atoms, and wherein R₁ and R₂ may be the same or different, and wherein E is a moiety selected from the group consisting of ##STR4##

The following examples illustrate the invention:

EXAMPLE I Preparation of 2,7-dibromo-9,9-di-n-decyl-9H-fluorene

Fluorene (27.70 g, 0.016 mol) and hexane (620 ml) were added to a three-necked round bottom flask equipped with a reflux condenser, a mechanical stirrer, and an addition funnel containing n-butyllithium (100 ml of a 2.0M solution in cyclohexane, 0.2 mol). The fluorene was dissolved in hexane by stirring and warming the solution with a heat gun. When all the fluorene was in solution, the solution was allowed to cool to room temperature. The butyllithium solution was then added dropwise over a 45 min period at room temperature. The rust-colored solution was stirred an additional hour before adding N, N, N', N',-tetramethyleneethylenediamine (TMEDA) (23.20 g, 0.2 mol) dropwise through a clean addition funnel. The solution was stirred an additional hour before 1-bromodecane (44.24 g, 0.2 mol) was added through another clean addition funnel. The pale orange solution was refluxed for 4 hours and then cooled to room temperature using an ice bath. A second equivalent of butyllithium (100.0 ml of a 2.0M solution in cyclohexane, 0.2 mol) was added at room temperature and was allowed to stir for an additional hour. Finally, 1-bromodecane (44.24 g, 0.2 mol) was added dropwise and the solution heated at reflux for an additional 6 hours.

The solution was cooled to room temperature and the resulting solid was vacuum filtered and the crystals rinsed with hexane. The solvent was removed under reduced pressure to yield a yellow oil that was purified by column chromatography on silica gel using hexane as the eluent. The product was futher purified by vacuum distillation (bp 257° C. (2 Torr)) to remove any excess bromodecane to afford 9,9-di-n-decyl-9H-fluorene as clear yellow oil in 93% yield. Mass Spec. m/z 446 (M⁺), 418 (M--C₂ H₄), 305 (M--C₁₀ H₂₁). Elemental Analysis: Calculated for C₃₃ H₅₀ : C, 88.72; H, 11.28. Found: C, 88.47; H, 10.93.

A solution of 9,9-di-n-decyl-9H-fluorene (21.33 g, 0.047 mol), iodine (0.12 g, 0.47 mmol), and methylene chloride (170 ml) was stirred magnetically in a single-necked round bottom flask covered with aluminum foil to exclude light. Elemental bromine (5.18 ml, 0.10 mol) in methylene chloride (20 ml) was pipetted into an addition funnel which was then added dropwise to the reaction mixture over a period of 15 minutes. Residual bromine solution was rinsed out of the addition funnel into the reaction mixture using 20 ml of additional methylene chloride and the reaction was allowed to stir for 20 hours at room temperature. An aqueous solution (15% by weight) of sodium bisulfite (NaHSO₃) was then added to the reaction mixture and the resulting two phases allowed to stir for 30 min. The organic layer was then separated and washed 3 times with an equal volume of water and then dried over anhydrous magnesium sulfate. The solution was filtered, concentrated and the resulting solid recrystallized from absolute ethanol to yield 2,7-dibromo-9,9-di-n-decyl-9H-fluorene as to white crystals, mp 38.1°-38.4° C. in 82.8% yield. Mass Spec. m/z 602, 604, 606 (M⁺), 461, 463, 465 (M--C₁₀ H₂₁), 382, 384 (461-Br). Elemental Analysis: Calculated for C₃₃ H₄₈ Br₂ : C, 65.56; H, 8.00; Br, 26.44. Found: C, 64.99; H, 8.21; Br, 27.25.

EXAMPLE II Preparation of 2,7-bis-(2-thienyl)-9,9-di-(n-decyl)-9H-fluorene

2-(tributylstannyl)thiophene (5.80 ml, 0.018 mol) was added under nitrogen directly to a single-necked round bottom flask equipped with a condenser and a magnetic stir bar that had been previously dried in an oven at 110° C. The 2,7-dibromo-9,9-di-n-decyl-9H-fluorene (5.26 g, 8.70 mmol) was weighed into a beaker and dissolved in toluene (14 ml) which had been freshly distilled and degassed with nitrogen. The resulting fluorene solution was pipetted into the round bottom flask and the beaker rinsed twice with 3 ml portions of degassed toluene. Pd(PPh₃)₄ (0.50 g, 0.43 mmol) and PdCl₂ (PPh₃)₂ (0.31 g, 0.44 mmol) were added to the reaction flask and the solution refluxed until it turned black. The toluene was removed under reduced pressure and the residue dissolved in hexane. The hexane solution was stirred vigorously with a 100 ml of a 2% aqueous KF solution for 3 hours. The organic layer was separated, washed twice with an equal volume of water, dried over anhydrous MgSO₄, filtered and concentrated. The crude product was purified by column chromotography on silica gel using hexane as the eluent. The purified waxy solid compound (mp 59.9°-60.8° C.) was isolated in a 70% yield. Mass Spec. m/z 610 (M⁺), 469 (M--C₁₀ H₂₁), Elemental Analysis: Calculated for C₄₁ H₅₂ S₂ : C, 80.60, H, 8.91, S, 10.49. Found: C, 80.55, H, 8.97, S, 10.08.

EXAMPLE III 3,4-Bis(decyloxy)-5-(benzothiazol-2-yl)thiophene-2-carboxylic acid

A mixture of the 3,4-bis(decyloxy)-2,5-thiophene dicarboxyllic acid (23.5 g), dichlorobenzene (130 ml), 2-aminothiophenol (8.9 g), trimethylsilylpolyphosphate (60 g), and tri-n-butylamine was heated at 100° C. for 22 hr under nitrogen. After cooling, the reaction mixture was diluted with water, the chlorobenzene layer separated, dried over magnesium sulfate and concentrated. The residual waxy solid was chromatographed over 150 g of silica gel. Elution with heptane gave the bis-benzothiazole by-product. 12.74 g (40% yield), mp 76°-77.8° C. Futher elution with toluene-heptane mixtures(1:3 and 1:2) gave the desired monocarboxyllic acid which was recrystallized from toluene-heptane to give 11.27 g, (40.5% yield), mp 139.7°-141.7° C.

EXAMPLE IV 2-Bromo-3,4-bis(decyloxy)-5-(benzothiazol-2-yl)-thiophene

To a magnetically stirred mixture of the monocarboxyllic acid (17.2 g), potassium carbonate (8.4 g), and water (300 ml) held at 45° C., bromine (8.4 g) was added over 15 min. After 45 min at 50° C. the mixture was cooled to room temperature and treated with 10% sodium hydroxide, water, and saturated sodium chloride solution. After drying, the toluene solution was concentrated and the residual oil was chromatographed over 200 g of silica gel. Elution with 1:3 toluene-heptane gave an oily product which solidified upon standing with methanol to give 16.85 g (91% yield), to mp 42°-43° C. Mass Spec: m/z 480-482 (M⁺ --C₉ H₁₉). Elemental Analysis: Calculated for C₃₁ H₄₆ BrNO₂ S₂ : C, 61.16, H, 7.62, Br, 13.14, N, 2.30, and S, 10.51%. Found: C, 60.85, H, 7.70, Br, 13.48, N, 2.29, and S, 10.42 %.

EXAMPLE V 3,4,3',4'-tetradecyloxy-5,5'-bis(benzothiazol-2-yl)-2,2'-bithiophene

A solution of 2-bromo-3,4-bis(decyloxy)-5-(benzothiazol-2-yl)-thiophene (12.2 g 20 mmol), and hexabutyl ditin (5.8 g, 10 mmol) in dioxane (100 ml), (freshly distilled from sodium after treatment with potassium hydroxide), was degassed by bubbling nitrogen through the solution for 20 min. Under nitrogen, tetrakistriphenylphosphinopalladium (0.468 g) was added and the mixture was heated at 100° C. for 20 hr. After cooling, pyridine (5 ml) was added and the resulting mixture allowed to stand for 1 hr. The separated solids were filtered and the filtrate diluted with an equal volume of water. The mixture was extracted with an equal volume of toluene, dried over magnesium sulfate and concentrated. The residue was combined with the initial solids and chromatographed over 250 g of silica gel. Elution with 3:1 and 2:1 heptane:toluene gave the desired product which was recrystallized from heptane to give 7.02 g (66.5% yield) mp 93.9°-95° C. Mass Spec. m/z 916 (M⁺ --C₁₀ H₂₀). Elemental Analysis: Calculated for C₆₂ H₉₂ N₂ O₄ S₄ : C, 70.36, H, 8.77, N, 2.65, and S, 12.10%. Found: C, 69.98, H, 8.90, N, 2.59, and S, 11.98 %.

EXAMPLE VI 2,7-bis(4-pyridyl)-9,9-di-n-decyl-9H-fluorene

4-(tri-n-butylstannyl)pyridine (8.76 g, 23.8 mmol) was added directly to a single-necked round bottom flask equipped with a condenser and a magnetic stir bar that had been dried in an oven at 110° C. overnight. 2,7-dibromo-9,9-di-n-decyl-9H-fluorene (6.85 g, 11.33 mmol) was weighed into a beaker and dissolved in dry toluene (30 ml) degassed with nitrogen. The fluorene solution was pipetted into the round bottom flask and the beaker rinsed with 25 ml portions of degassed toluene. Pd(PPh₃)₄ (0.28 g, 0.24 mmol), PdCl₂ (PPh₃)₄ (1.67 g, 2.38 mmol) and triphenylphosphine (1.37 g, 5.22 mmol) were added to the reaction flask and the solution heated at reflux until it turned black. The reaction mixture was cooled to room temperature, the toluene removed under reduced pressure and the residue dissolved in hexane. The hexane solution was stirred vigorously with a 100 ml of a 2% aqueous KF solution for 3 hours. The organic layer was separated, washed twice with an equal volume of water, dried over anhydrous MgSO₄, filtered and concentrated. The crude product was purified by column chromatography on silica gel using 30/70 THF/hexane. The purified compound was isolated in 18.7% yield as light brown oil. Mass Spec. m/z 600 (M⁺), 459 (M--C₁₀ H₂₁), 167 (C₁₁ H₇ N₂)+. Elemental Analysis: Calculated for C₄₃ H₅₆ N₂ : C, 85.95, H, 9.39, N, 4.66. Found: C, 85.59, H, 8.97, N, 4.16.

EXAMPLE VII 4,4'-(9,9-bis-n-decyl-2,7-fluorenediyldi-2,1 -ethenediyl)bis-pyridine

2,7-Dibromo-9,9-di-n-decyl-9H-fluorene (10.0 g, 16.5 mmol), tri-o-tolylphosphine (2.01 g, 6.6 mmol), degassed triethylamine (40 ml) and 4-vinylpyridine (5.35 ml, 49.62 mmol) was added to a single-necked round bottom flask under to nitrogen equipped with a magnetic stir bar and a reflux condenser. The palladium catalyst (0.19 g, 0.83 mmol) was added and the solution heated at 95° C. for 18 hours. The reaction mixture was cooled to room temperature and the solvent removed under reduced pressure. The residue was dissolved in methylene chloride, and the organic layer washed three times with equal volumes of water, dried over anhydrous MgSO₄, filtered and concentrated. The product was purified by column chromotography on alumina using 30/70 ethyl acetate/hexane to yield the product as a yellow oil in 71.48% yield.Mass Spec. m/z 652 (M⁺), 511 (M--C₁₀ H₂₁). Elemental Analysis: Calculated for C₄₇ H₆₀ N₂ : C, 86.45, H, 9.26, N, 4.29. Found: C, 86.50, H, 8.84, N, 3.78.

EXAMPLE VIII Determination of 2-Photon Absorption Cross-Section

The two photon absorption coefficients β and the molecular two-photon cross-section σ₂ were determined from an experimental measurement of the transmitted intensity of a laser beam at 798 nm as a function of the incident intensity. According to the basic theoretical consideration, the two-photon absorption (TPA) induced decrease in transmissivity can be expressed as

    I(L)=I.sub.O /(1+I.sub.O Lβ)                          (1)

where I(L) is the transmitted beam intensity, I_(O) the incident beam intensity, L the thickness of the sample, and β is the TPA coefficient of the sample medium. In the to derivation of the above equation it is assumed that the linear attenuation of the medium can be neglected and the beam has a nearly uniform transverse intensity distribution within the medium. The TPA coefficient can be determined by measuring the transmitted intensity I(L) as a function of various incident intensities I_(O) for a given medium with a given L value. The TPA coefficient β (in units of cm/GW) of a given sample is determined by

    β=σ.sub.2 N.sub.O =σ.sub.2 N.sub.A d.sub.0 ×10.sup.-3(2)

where N_(O) is the molecular density of the material being measured (in units of 1/cm³), (σ₂ the molecular TPA cross-section of the same material (in units of cm⁴ /GW), d₀ is the concentration of the material (in units of M/L) and finally N_(A), Avogadro's number. For known β and d₀, the value of σ₂ can be easily calculated from equation (2).

In practice, various optical detectors such as photodiodes, photomultipliers, photometers and optical power meters can be used to measure the incident beam intensity I_(O) and the transmitted beam intensity I(L) separately. The change of the I_(O) values can be done by using variable optical attenuators (such as neutral filters or rotatable polarizing prisms), or by varying the beam cross-section of the input laser beam (by changing the relative distance of the sample to the focal point of the input beam). The input laser beam used for the experimental measurements was a pulsed laser dye system with a wavelength of 800 nm, a spectral width of 1-10 nm, a pulse duration of 8-10 ns, a beam size (before focusing lens) of 3-5 nm, a beam divergency of 1.2-1.5 mrad, and a repetition rate between 1-30 Hz.

    ______________________________________     Experimentally Determined TPA coefficients (β) and cross-sections     (σ)     Compound             Concentration                        β   σ.sub.2     of Example             M/L        cm/GW    (×10.sup.-20) cm.sup.4 /GW)                                            Solvent     ______________________________________     II      0.0418     0.058    0.230      THF     V       0.0567     5.000    14.600     THF     VI      0.0935     0.085    0.152      THF     VII     0.0935     0.491    0.873      THF     ______________________________________

These data clearly show increased size of the 2-photon cross-section as compared to most state-of-the-art organic compounds. The increased cross-section, when coupled with strong upconverted fluorescence, makes these chromophores more useful in fluorescent imaging application such as 2-photon laser scanning confocal microscopy where strong fluorescence is needed to obtain high resolution. Large 2-photon absorption cross-sections also lead to greatly improved optical limiting behavior. In addition the long chain alkyl groups incorporated into these materials lead to very high solubility in organic solvents and good compatibility with organic polymers. Solubility of these chromophores is generally at least 0.01 and up to about 35% in common organic solvents.

Two-photon absorbing dyes which exhibit upconverted fluorescence have also found use in other areas of photonic technology. These include 2-photon pumped upconverted lasing, 2-photon confocal microscopy, 2-photon photodynamic therapy, 2-photon optical power limiting, and 3D optical data storage.

Various modifications may be made in the instant invention without departing from the spirit and scope of the appended claims. 

We claim:
 1. A two-photon absorbing chromophore of the formula

    E--Ar--E

wherein the Ar core is ##STR5## wherein R₁ and R₂ are alkyl groups having 8 to 12 carbon atoms, and wherein R₁ and R₂ are the same or different, and wherein E is a moiety selected from the group consisting of ##STR6##
 2. The compound of claim 1 wherein Ar is ##STR7## wherein R₁ and R₂ are n-decyl, and E is ##STR8##
 3. The compound of claim 1 wherein Ar is ##STR9## wherein R₁ and R₂ are n-decyl, and E is ##STR10## 